1. Field of the Invention
The present invention relates to an apparatus and method for production of a porous optical fiber preform, more particularly relates to an apparatus and method for production of a porous optical fiber preform for preventing cracking and maintaining a uniform quality of the porous optical fiber preform when producing a porous optical fiber preform by the VAD method.
2. Description of the Related Art
Optical fibers are being used in various fields such as optical communications. Optical fibers are, briefly, produced by the following method. First, a porous optical fiber preform is formed. Next, the porous optical fiber preform is heated and vitrified. Then, the vitrified optical fiber preform is heated and drawn to form a single mode optical fiber comprised of for example a core of a diameter of 10 xcexcm and a cladding of a diameter of 125 xcexcm formed at the circumference of the core. The thus formed optical fiber is then covered by a resin.
The VAD method is mainly used for producing the porous optical fiber preform used for producing the optical fiber in this way. A porous optical fiber preform is produced by the VAD method as shown, for example, in FIG. 1 illustrating the interior of a reaction vessel 11, by charging silicon tetrachloride (SiCl4) gas into oxyhydrogen flames 3 emitted using a core forming multitube burner 1 and a cladding forming multitube burner 2 arranged in a reaction portion 11C of the reaction vessel 11 so as to generate fine particles of glass, that is, silicon dioxide (SiO2), by a flame hydrolysis reaction, making the fine particles of glass deposit on a rotating seed rod 4, and thereby form a fine glass particle deposit 5 around the seed rod 4. The rotating seed rod 4 is pulled up so that the fine glass particle deposit 5 is formed in its longitudinal direction and thereby obtain a porous optical fiber preform. At this time, if charging a small amount of fine particles of germanium chloride (GeCl2) etc. into the core forming multitube burner 1 along with the silicon tetrachloride as a dopant for raising the refractive index of the core portion from the cladding portion, fine particles of germanium oxide (GeO2) etc. are simultaneously produced and a spread of germanium dioxide can be created in the radial direction of the fine glass particle deposit 5. Note that reference numeral 10 shows a feed port, 13 a main exhaust port, and 12 a secondary exhaust port.
In the above method, the fine particles of glass are deposited on the seed rod 4 in a horizontal stream of gas 6 from the burner 1 and 2 side to the secondary exhaust port 12 side while rotating and pulling up the seed rod 4. The not deposited excess fine particles of glass are carried by the horizontal stream of gas 6 introduced from the feed port 10 through the reaction portion 11C to the main exhaust port 13 where they are exhausted to the outside of the reaction vessel 11. By exhausting the excess fine particles of glass not deposited on the seed rod 4 in this way, it is possible to prevent the excess fine particles of glass not deposited on the seed rod 4 from depositing on the reaction vessel 11 and then peeling off from the reaction vessel 11 and depositing on the surface of the fine glass particle deposit 5 and thereby causing bubbles in the fine glass particle deposit 5 after vitrification.
As the horizontal stream of gas 6, air or an inert gas such as argon gas is used. The horizontal stream of gas 6 is usually introduced in an ordinary temperature state.
When forming a fine glass particle deposit by the VAD method, however, the soot growth rate, that is, the distance by which the seed rod (deposit of fine particles of glass) is pulled up per unit time, must be made constant in order to prevent fluctuations in the cutoff wavelength, to obtain a uniform shape of the fine glass particle deposit, etc. That is, if the flames can be kept steady without flickering, the rate of growth of the fine glass particle deposit becomes stable and the distribution of the dopant for raising the refractive index of the core portion, for example, the germanium, in the longitudinal direction of the fine glass particle deposit becomes constant so the final optical fiber ends up with less variations in characteristics.
In practice, however, disturbances in the flow of gas in the reaction vessel make the flames flicker and therefore it is difficult to make the growth rate in the longitudinal direction of the fine glass particle deposit constant. Therefore, various measures have been devised to prevent flickering of the flames.
As one example, to prevent the flow of horizontal stream of gas 6 from being disturbed at the portion where the fine glass particle deposit 5 is formed on the seed rod 4, the method has been proposed of stabilizing the flow of the horizontal stream of gas 6 by making the horizontal stream of gas 6 forcibly flow by a blower (see Japanese Unexamined Patent Publication (Kokai) No. 1-242431).
As another example, the method has been proposed of providing a baffle plate surrounding the main exhaust port 13 so that the horizontal stream of gas is exhausted smoothly from the main exhaust port 13 (see Japanese Unexamined Patent Publication (Kokai) No. 2-2836321).
These methods however have had the following problems.
The first problem is that the horizontal stream of gas 6 strikes the fine glass particle deposit while it is being formed. The horizontal stream of gas 6 introduced at an ordinary temperature causes the surface temperature of the fine glass particle deposit 5 to fall, so the density of the fine glass particle deposit 5 falls and cracks form In the fine glass particle deposit 5.
The second problem is that the horizontal stream of gas 6 strikes the flames of the burners 1 and 2. The flames of the burners 1 and 2 are made to flicker and therefore the quality of the fine glass particle deposit 5 being formed, in other words the quality of the porous optical fiber preform, fluctuates in the longitudinal direction.
An object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the horizontal stream of gas from the feed port to the main exhaust port from disturbing the shapes of the flames carrying fine particles of glass and prevents it from contacting the portion where the fine glass particle deposit is being formed.
Another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the surface temperature of the fine glass particle deposit from falling and thereby prevents cracks in the fine glass particle deposit.
Still another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which prevents the flames carrying the fine particles of glass from the burners from flickering and thereby prevents fluctuations in the quality of the fine glass particle deposit (porous optical fiber preform) in the longitudinal direction.
Still another object of the present invention is to provide an apparatus for the production of a porous optical fiber preform which enables fine particles of glass emitted from the burners to efficiently deposit on the seed rod and thereby efficiently form a fine glass particle deposit (porous optical fiber preform).
Still another object of the present invention is to provide a method for production of the above porous optical fiber preform.
According to a first aspect of the present invention, there is provided an apparatus for the production of a porous optical fiber preform having a core portion and a cladding portion formed at the circumference of the core portion, the apparatus for the production of a porous optical fiber preform provided with a reaction vessel having a reaction portion, a feed port for introducing a stream of gas into the reaction portion, and a main exhaust port, facing the feed port in a horizontal direction across the reaction portion, for exhausting the gas from the reaction portion; a rotating means, introduced into the reaction portion, for mounting, rotating, and pulling up a seed rod for forming the porous optical fiber preform; a first burner, introduced into the reaction portion, for emitting a flame carrying fine particles of glass forming the core portion in the horizontal direction or upward toward the seed rod mounted to the rotating means or the porous optical fiber preform deposited on the seed rod; a second burner, introduced into the reaction portion, for emitting a flame carrying fine particles of glass forming the cladding portion in the horizontal direction or upward on to the core portion of the porous optical fiber preform deposited on the seed rod mounted at the rotating means; and a flow adjusting means for adjusting the flow of the stream of gas from the feed port to the main exhaust port so that the flames carrying the glass particles emitted from the first and second burners to be deposited on the seed rod are not disturbed by the stream of gas and so that the stream of gas does not directly contact a fine glass particle deposit formed at a bottom end of the seed rod.
Preferably, the reaction vessel further has a secondary exhaust port, positioned between a portion forming the porous optical fiber preform in the reaction portion and the main exhaust port, for exhausting the gas behind the porous optical fiber preform to the outside.
Alternatively or more preferably, the flow adjusting means adjusts the flow of the stream of gas so as to be higher than a bottom end area of the porous optical fiber preform at which the fine particles of glass emitted from the first and second burners are deposited on the seed rod.
Still more preferably, the flow adjusting means includes a baffle plate provided at the feed port so as to partially block the lower part of the feed port and make the stream of gas flow higher than a bottom end area of the porous optical fiber preform at which the fine particles of glass emitted from the first and second burners are deposited on the seed rod.
Even more preferably, the flow adjusting means further has a guide rail for making the baffle plate move up and down in a vertical direction in the feed port.
In the first aspect, again alternatively or more preferably, the flow adjusting means includes two baffle plates arranged at a center of two feed ports provided together so as to make the stream of gas flow along side walls of the reaction portion in the horizontal direction and not flow to the positions of provision of the first and second burners.
Here, still more preferably, the flow adjusting means further has guide rails for making the two baffle plates move in a horizontal direction at the feed ports.
In the first aspect, again alternatively or more preferably, the flow adjusting means includes a baffle plate with a width in the horizontal direction larger than a diameter of the porous optical fiber preform to be formed on the seed rod.
Here, still more preferably, the flow adjusting means is formed with a hole through which the second burner passes and the second burner is arranged to pass through the hole of the flow adjusting means and emit the flame carrying the fine particles of glass forming the cladding portion upward on to the fine particles of glass forming the core portion deposited on the seed rod.
Here, even more preferably, the baffle plate is flat.
Here, alternatively even more preferably, the baffle plate is shaped bent in the horizontal direction and flat in the vertical direction.
In the latter case, more preferably the baffle plate is provided with a plurality of small holes arranged in the vertical direction near the two ends in the horizontal direction.
According to a second aspect of the present invention, there is provided a method for the production of a porous optical fiber preform having a core portion and a cladding portion formed at the circumference of the core portion, the method for the production of a porous optical fiber preform comprising adjusting the flow of a stream of gas from a feed port through a reaction vessel toward a main exhaust port so that a first flame carrying fine particles of glass forming the core portion and a second flame carrying fine particles of glass forming the cladding portion are not disturbed by the stream of gas.
Preferably, the method further comprises adjusting the flow of the stream of gas so as to be higher than a bottom end area of the porous optical fiber preform deposited on the seed rod.
More preferably, the method further comprises partially blocking the lower part of the feed port and making the stream of gas flow higher than a bottom end area of the porous optical fiber preform deposited on the seed rod.
Still more preferably, the method further comprises making the stream of gas flow along side walls of the reaction portion in the horizontal direction so as not to disturb the first and second flames carrying the fine particles of glass.
That is, in the present invention, the flow adjusting means prevents the horizontal stream of gas flowing from the feed port to the main exhaust port from disturbing the shapes of the flames carrying the fine particles of glass and contacting the fine glass particle deposit. As a result, it is possible to prevent the surface temperature of the fine glass particle deposit from falling and cracks from forming in the fine glass particle deposit. Further, it is possible to prevent the flames carrying the fine particles of glass from the burners from flickering and therefore prevent fluctuations in the quality of the fine glass particle deposit (porous optical fiber preform) in the longitudinal direction.